A screw conveyor has a feed screw rotatably mounted in a fixed tube. The tube itself is provided at some point with a feed opening of a certain size. A screw conveyor of this type conveys material in the form of dust or granules with a high degree of regularity, so that it can also be used as a metering device. When small amounts are to be conveyed, i.e. on the order of 50 g/hr or approximately 1 g/min, only small amounts of the material to be conveyed are present between two screw threads. This results in a batchwise feed, so that metering is constant over long time intervals but periodic fluctuations of considerable amplitude occur within short intervals, with the result that a screw conveyor cannot be used for fine metering in situations in which a high degree of constancy of the delivery, i.e. the amount of material conveyed per unit time, is required for small amounts to be conveyed not only over long time intervals but also over short time spans as well.